1. Field of the Invention
The present invention relates to a diversity radio communications systems for multipath fading channels, using a spread spectrum multiplexing technique.
2. Description of the Related Art
In radio communications systems where frequency-selective, multipath fading occurs, the waveform of transmitted symbols is affected by the time dispersal effect of the fading channel. Diversity reception and adaptive equalization techniques are normally employed to combat this problem. One of the known techniques is the frequency-and-time diversity reception wherein an intermediate-frequency modulated information-bearing signal and a delayed replica of this signal are up-converted to two radio signals of different frequencies and transmitted from a single antenna. At a receive site, the transmitted signals are demultiplexed onto separate diversity branches where they are low-noise amplified and down-converted to IF signals from which baseband signals are recovered. One of the baseband signals is delayed so that they are time coincident with each other and then compared with each other. One of the signals that is less distorted is selected as an input to an adaptive equalizer where multipath-fading related intersymbol interference is canceled. Alternatively, the time-aligned signals are modified both in phase and amplitude so that they are maximal-ratio combined to produce an input to the adaptive equalizer. However, due to the use of two radio frequencies, the cost of high-power transmitters and low-noise receivers is substantial if the number of diversity branches increases.
Another prior art approach is concerned with the space-time diversity reception technique. As described in Japanese Provisional Patent Publication Sho-63-286027, a radio-frequency modulated signal and a delayed replica of this signal are transmitted from respective antennas which are spaced so that the transmitted signals propagate through separate multipath Rayleigh fading channels and received by a single antenna at a receive site, where the received signals are down-converted to baseband signals with a differential delay between them. These signals are diversity combined in a RAKE equalizer. While the cost of the receive site is reduced by the use of a single antenna, the use of two antennas at the transmit site would add an extra cost if the system is to be used in a microwave communications where large aperture antennas are required.